1. Field of the Invention
The present invention relates to a pervaporation method of selectively separating water from an organic material aqueous solution through an aromatic imide polymer asymmetric membrane.
More particularly, the present invention relates to a pervaporation method of selectively separating water from an organic material aqueous solution through an aromatic imide polymer asymmetric membrane at a high selectivity and at a high permeation rate, to concentrate or collect the organic material.
2. Description of the Related Art
It is known that water can be separated from an aqueous solution of an organic material by a distillation method. In the distillation method, however, some of the organic materials form an azeotropic mixture, or have boiling points close to water, or are chemically modified at the distillation temperature, and therefore, it is very difficult to selectively separate water from the organic materials.
To avoid the above difficulty, a method of separating water from an organic material by using a separating membrane has been attempted. In this method wherein a membrane is used to separate or concentrate an organic material aqueous solution, an aqueous solution of organic material is brought into contact with a face of a membrane, to allow a specific liquid to selectively permeate through the membrane due to a differential osmotic pressure. This method is referred to as a reverse osmosis method.
Usually, in the reverse osmosis method for an organic material aqueous solution, a higher pressure than the osmotic pressure of the aqueous solution must be applied to the aqueous solution, and therefore, the reverse osmosis method can not be applied to a concentrated organic material aqueous solution which exhibits a high osmotic pressure, and accordingly, the reverse osmosis method can be applied only to organic material aqueous solutions having a limited concentration of the organic material.
Recently, as a new type of separating method in which a separating membrane is used, a pervaporation method has been developed for an organic material solution and is now under serious consideration in this field.
In the pervaporation method, an organic material aqueous solution in the state of a liquid is brought into direct contact with a feed side face of a separating membrane capable of selectively allowing a specific component, for example, water, to permeate therethrough, and the opposite delivery side face of the membrane is exposed to a vacuum or a reduced pressure. The specific component, for example, water, is allowed to selectively permeate through the membrane and is collected in the state of a vapor at the opposite delivery side of the membrane. This method is useful for selectively separating or concentrating an individual organic material from an aqueous solution thereof compound mixture.
Also, it is known that aromatic imide polymers exhibit a higher heat resistance and chemical resistance than those of conventional polyamides, cellulose and cellulose acetate, and thus the utilization of the aromatic imide polymer as a material for forming a separating membrane is now under serious consideration. Accordingly, attempts have been made to utilize an asymmetric membrane consisting of a conventional aromatic imide polymer for separating water in the vapor state from an aqueous solution of an organic material.
Nevertheless, when used for the pervaporation method of separating water from an organic material aqueous solution, the conventional aromatic imide polymer separating membranes are disadvantageous in that:
(1) when the contact of the aromatic imide polymer membrane with the organic material aqueous solution or water is kept at a high temperature for a long time, the separating performance and mechanical strength of the membrane are significantly decreased, because the aromatic imide polymer in the membrane is decomposed by a hydrolysis thereof; PA1 (2) due to the above-mentioned decomposition of the polymer, the aromatic imide polymer membrane exhibits a decreased durability, and thus does not always provide a satisfactory separation of water from the organic material aqueous solution; and PA1 (3) ,the selective separation of water from the organic material aqueous solution and the permeating rate of water through the conventional aromatic imide polymer membrane are not satisfactory in the pervaporation separating method. PA1 (A) an aromatic tetracarboxylic acid component comprising: PA1 (B) an aromatic diamine component comprising:
Accordingly, there is a strong demand for a new type of aromatic imide polymer asymmetric membrane useful for the pervaporation method of selectively separating water from an organic material aqueous solution.